Effect of solution chemistry on the iodine release from iodoapatite in aqueous environments

Abstract

To ensure the safe disposal of nuclear waste, understanding the release process of radionuclides retained in the nuclear waste forms is of vital importance. Iodoapatite Pb9.85(VO4)6I1.7, a potential waste form for iodine-129, was selected as a model system for ceramic waste forms in this study to understand the effect of aqueous species on iodine release. Semi-dynamic leaching tests were conducted on bulk samples in cap-sealed Teflon vessels with 0.1 mol/L NaCl, Na2CO3, Na3PO4, and Na2SO4 solutions under 90 °C, fixed sample surface area to solution volume ratio of 5/m, and periodic replacement of leaching solutions. The reacted solutions were then analyzed by Inductively Coupled Plasma-Mass Spectrometry and Inductively Coupled Plasma-Optical Emission Spectrometry; the leached surfaces were characterized by X-ray diffraction, scanning electron microscopy, and infrared spectroscopy. The result shows that, compared to deionized water, the ion-rich solutions enhanced the iodine release as a result of the increased ionic strength, reduced activity coefficient of dissolved species, and increased solution pH. Surface reactions can lead to the formations of secondary phases by ion-exchange and precipitation. These findings suggest that an ion-rich environment in the geological repository can be detrimental to the disposal safety of the apatite waste form.

Corrosion interactions between stainless steel and lead vanado-iodoapatite nuclear waste form part I

“…Cl only exists inside the crevice, most likely as Pb 5 (VO 4 ) 3 Cl due to its low solubility. These results further supported the notion that the corrosion of I-APT is primarily through the ion exchange between I − and Cl − 11,37 , since Pb 5 (VO 4 ) 3 Cl is the dominant ion-exchange product of I-APT in NaCl solutions 11 . For the area outside the crevice, the V and Pb peaks both varied slightly (~0.1 eV) compared to those from the area inside the crevice.…”

“…In this study, the corrosion interactions between SS 316 and lead vanado-iodoapatite (I-APT, Pb 9.85 (VO 4 ) 6 I 1.7 ), a potential I-129 waste form [5][6][7] , are explored. The corrosion mechanism of I-APT has been reported in previous studies 5,11 . In general, when I-APT is exposed to an aqueous environment, the I − ions within the I-APT matrix will exchange with other anions (e.g., OH − ) in the leaching solutions.…”

“…This may be associated with the precipitation of Cl-rich apatites during the corrosion of I-APT, as supported by the strong Cl signal in the EDS elemental map. Studies have shown that Pb 5 (VO 4 ) 3 Cl is the primary corrosion product of I-APT in NaCl solutions 11 . The precipitation of I-rich apatite is less favorable in solutions containing Cl − ions due to the ion-exchange corrosion mechanism.…”

Thermodynamic equilibrium and kinetic fundamentals of oxide dissolution in aqueous solution

“…For apatite, experimental results show that the dissolution rate of natural apatite (Ca 10 (PO 4 ) 6 F 2 ) is approximately 3 orders of magnitude lower than that of synthetic iodoapatite (Pb 10 (VO 4 ) 6 I 2 ) with the same structure at a given temperature and pH. The activation energy for the dissolution of natural apatite is more than 2 times that of iodoapatite [29,30,31]. The higher ionic potentials of the constituent elements in natural apatite lead to shorter bond distances and stronger chemical bonding with respect to iodoapatite, which result in a higher activation energy and lower dissolution rate for fluorapatite than iodoapatite [31] ], are incompatible with the network structure of borosilicate glass, leading to their low solubility in glass [32].…”

Corrosion interactions between stainless steel and lead vanado-iodoapatite nuclear waste form part II

“…This leads to the formation of a highly reactive zone at the crevice mouth, which in turn affects the corrosion rate of I-APT that is present nearby. It has been known that the release rate of iodine from I-APT depends on the salt concentration 32 . Therefore, the increased alteration layer thickness on I-APT, along with the precipitation of large crystals in the SS/I-APT crevice mouth area, can be well explained by this mechanism.…”